The present application is based on Japanese Application 317169/2000, filed Oct. 17, 2000, which is herein incorporated by reference in its entirety.
1. Field of the Invention
The present invention relates to a boiling water reactor nuclear power plant with a natural circulation reactor, and particularly to a boiling water reactor nuclear power plant and its construction method with a simplified compact system configuration, having diversity to the power demand, and an improved economy and similar.
2. Description of the Related Technology
In conventional plants, as the configuration of reactor pressure vessel and internals of the boiling water reactor nuclear power plant, forced circulation method has been mainly adopted such that active pumps installed at the bottom of the reactor pressure vessel circulates reactor water, and jet pumps installed in the reactor are driven by the external loop active pumps. And as the control rod of power control means, bottom entry type which is inserted from the bottom of reactor pressure vessel has been adopted.
For the above configuration, the lower drywell space has been large in order to keep the circulating pumps and the control rod drive mechanism and its draw out space below the reactor pressure vessel. And this lower drywell is used for a large quantity of water storage space during loss of coolant accident since the suppression pool water injected into the reactor pressure vessel is drawn down to this space from the break cavity. And therefore this space has been wasteful to design the water quantity of the suppression pool.
And if core fuel is melted and dropped down onto the bottom of the reactor pressure vessel during assumed severe accident, the configuration with control rod driving mechanism and similar prevents from cooling the melted core from outside the reactor vessel.
As for the example of the conventional plant among these boiling water reactor nuclear plants, it is explained using the newest type reactor named ABWR referring to FIG. 15 as follows.
On conventional boiling water reactor nuclear power plant, core shroud 3 which contains the core 2 is installed in the reactor pressure vessel 1, many fuel bundles 6 are provided between core support plate 4 and upper lattice plate 5 which is installed in each at lower and upper portion in the core shroud.
Shroud head 7 is installed above the core shroud 3, and steam separator 9 is installed above the shroud head 7 through the standpipes 8. Steam dryer 10 is installed above the steam separator 9. Control rod guide tubes 11 of the control rod (not figured) which are inserted into the core 2 and the control rod drive mechanism 12 which drives the control rods are installed below the core support plate 4.
Plural reactor internal pumps 13 are installed in the circumferential direction at the bottom of the reactor pressure vessel 1.
Main steam pipes 14 are provided on the sidewall of the reactor pressure vessel 1 alongside the steam dryer 10 to supply reactor steam to the turbine (not shown). Feed water pipes 15 are provided on the sidewall of the reactor pressure vessel 1 alongside the standpipes 8 of steam separators to supply cooling water to this reactor 1.
In the boiling water reactor of this configuration, the coolant above the core is drawn into the internal pumps 13 from the gap area between the core shroud 3 and the reactor pressure vessel 1, and it generates steam in the core 2 through the core bottom, and it is lead to the turbine through the main steam pipes 14 via. the standpipes 8, the steam separators 9 and the steam dryer 10. The steam which works in the turbine is condensed by the main condenser, and the condensed water is returned to the reactor upper portion through the feed water pipes 15.
In these conventional type boiling water reactor nuclear power plants, the core and the pressure containment vessel is cooled during accident using active components in general, and for the severe accident means, in which these active components are assumed not to be used, passive cooling systems or alternate active cooling/injection systems have been added at present.
Therefore it requires additional exclusive passive equipments for the severe accident other than the design base accident equipments and system regardless of degree of these reliability and it has been given a large impact economically.
And as for the cooling method inside drywell during reactor normal operation, cooling heat exchanger (drywell cooler), for which cooling water is supplied from outside drywell, and fans for circulating the inner gas are installed in the drywell in general.
The conventional pressure containment vessel is made from steel or reinforced concrete. Namely, a gourd-shaped standalone type or a bell-shaped integrated building type is adopted for the steel containment vessel. And an integrated dual-cylinder type is adopted for the reinforced concrete containment vessel. The vessel sizing factors are mainly as follows; components arrangement inside drywell as for drywell, water and air space volume for the pressure suppression in the early stage of accident as for suppression pool.
In addition the pressure containment vessel strength is evaluated if its stress is below the limit in which hydrogen gas generated by metal-water reaction is stored assumed severe accident condition continuing the heat removal using passive containment cooling heat exchanger or alternate cooling/injection system.
Reactor building is made from reinforced concrete separated from turbine building, and is designed on each site conditions or plant power output. These buildings are constructed almost on site works.
FIG. 16 shows an example of the latest boiling water reactor plant under studying.
This plant is constituted of reactor building 421, pressure containment vessel 422, reactor pressure vessel 423 and its associated system and component. In this constituted plant, control rod and its driving mechanism 435 is provided below the reactor core 424, and reactor coolant circulation components like reactor internal pumps 425 are provided on the reactor vessel bottom.
Emergency core cooling system pumps 426 are located on the lowest floor of the reactor building 421, and residual heat removal pumps 427 and heat exchangers 428 for the containment vessel cooling as same manner.
On severe accident which exceeds design base accident, core and containment vessel 422 is continuously cooled using passive containment cooling heat exchanger 429 and alternate injection pumps 430 via alternate injection water supply tank 436.
During annual plant inspection, core fuels are moved to spent fuel storage pool 432 on the condition of reactor well filled with water.
Although there has been previously existed the idea of forced circulation type boiling water reactor with internal type upper entry control rod drive mechanism, the concept of natural circulation type boiling water reactor with it combining raised type suppression pool has not known before.
In the case of a conventional system of bottom entry control rod and bottom located suppression pool, when gravity driven cooling system of passive safety is adopted, additional exclusive tank of gravity driven cooling system is needed on the upper elevation portion of pressure containment vessel other than suppression pool and it causes large volume of pressure containment vessel.
And bottom entry control rod mechanism causes large lower drywell volume for the flooding by gravity driven cooling system to become large pool capacity of gravity driven cooling system. Furthermore many various nozzles, piping and control rod guide tubes etc. located under the core prevents IVR (In Vessel Retention), which stops the progress this event, of the molten core at severe accident.
Meanwhile, the present inventors and others have investigated the whole reactor pressure vessel removal. But it is difficult for the conventional reactor pressure vessel since various piping below the core elevation can not be cut off with the core filled with coolant because of in vessel fuel condition.
If a natural circulation system is adopted, it requires large height chimney of two phase region to produce required circulation driving force to compensate the small driving force, and it causes large height of reactor pressure vessel. Furthermore if a high density core is adopted, it becomes unrealistic to get its required driving force on the natural circulation system since the narrow space of the two phase core region causes increased core pressure drop.
In a natural circulation reactor, there is no transient mitigation function corresponding to RPT (Re-circulation Pump Trip) of the current type boiling water reactor using forced circulation system, then ATWS (Anticipated Transient Without Scram) is more severe than forced circulation type reactor.
In the conventional boiling water reactor, there are significant economical problems of additional installation of some active and passive components to remove heat from the pressure containment vessel during a loss of coolant accident.
Moreover, assumed severe accident condition where the core fuel melts and falls down on the reactor pressure vessel bottom, then as for IVR (In Vessel Retention), the conventional boiling water reactor which has control rod guide tubes under the lower vessel mirror plate can be difficult for in vessel cooling from RPV outside wall, and as for the RPV core region gravity injection cooling from the lower drywell pool flooded by the suppression pool water it can be difficult for the sufficient cooling since the cooling water does not circulate sufficiently in the lower drywell pool and the RPV bottom wall surface is covered with the steam film generated.
Drywell coolers and their ducts in which the drywell gas is circulated are provided in the drywell, and it has severe arrangement space and needs to locate active components in it.
FIGS. 17 and 18 show examples of other conventional plant.
FIG. 17 shows the plant of raised suppression pool 433 type. FIG. 18 shows the plant of bottom located suppression pool 433 type.
In these plants active components such as pumps and fans are located in the relatively high radiation area of the drywell 434. During plant inspection maintenance workers have to enter in this area for the maintenance work, and this causes to increase their radiation exposure.
Furthermore when reactor pressure vessel 423 of approximately 20 m high, which is used forced circulation reactors, is applied for the raised suppression pool type pressure containment vessel 422, since main steam piping is arranged to the turbine through under the suppression pool 433 from upper portion of the reactor pressure vessel 423, it causes the increased piping material and drywell space to lead economic disadvantages
Since the building of a conventional boiling water reactor is generally constituted in respective building because of the different seismic design conditions of the components located in the reactor building and turbine building, it requires seismic and construction design respectively and it has an economical disadvantage for the increased floor space.
Furthermore since the design criteria (specification, seismic condition, etc.) for the reactor building is different in each construction site, it has been difficult for the design standardization. Moreover there has been problems of large increased works and period of the reactor building construction.
The present invention had been achieved for resolving the problems of the conventional technology, and the first target is to provide a compact and economic nuclear power plant.
The second target is to provide the volume reduced pressure containment vessel by maintaining the make up water in the suppression pool even if adopting the gravity driven cooling system of passive safety.
The third target is to minimize the lower drywell volume eliminating all the obstacles of nozzle and piping under the reactor pressure vessel, and the required pool volume to be injected by the gravity driven cooling system, so that IVR (In Vessel Retention of the molten core) can be easily carried out, on which the event progress is prevented in the reactor pressure vessel on severe accident.
The fourth target is to control the plant power to be suppressed in the ATWS (Anticipated Transient Without Scram) event until the boric acid solution injection system is initiated for the reactor shutdown.
The fifth target is to provide compact, simple and passive (natural force used) heat removal system from the containment vessel on a loss of coolant accident etc. to have high reliability and economical advantage.
The sixth target is to enable to cool the reactor pressure vessel wall on keeping the molten core in it through heat release outside the pressure containment vessel in case of core melt condition on severe accident to minimize the influence of severe accident for improving safety.
The seventh target is to provide maintenance free design and reduced required volume space in the drywell with no active component required.
The eighth target is to ensure that the molten core is isolated and kept cooling in the pressure containment vessel without active components in case of the core melted down and abnormal plant condition is reliably detected.
The ninth target is to enable the released heat in the drywell during accident to be transferred to the suppression pool without using active components, and to enable the event to be terminated without water flooding the lower drywell. And thereby the plant reliability can be improved.
The tenth target is to eliminate the required operator entrance in the drywell for the maintenance of valves etc. to reduce the radiation exposure.
The eleventh target is to standardize the seismic and the building design, and to significantly reduce the construction period to get economical advantage.
In order to achieve these targets, there may be provided a boiling water reactor nuclear power plant comprising: a reactor building; a pressure containment vessel positioned in the reactor building; a drywell comprising a space inside the pressure containment vessel; a pressure suppression pool provided inside the pressure containment vessel; a nuclear reactor pressure vessel contained by the pressure containment vessel; a reactor core having fuel assemblies supported by a reactor core support plate and an upper grid plate provided in an inner base portion of the nuclear reactor pressure vessel; a reactor core shroud surrounding the reactor core and the upper grid plate; control rod guide tubes positioned in the reactor core shroud and over the upper grid plate; control rods inserted in the control rod guide tubes; and control rod drive mechanisms which drive the insertion and withdrawal of the control rods from above the reactor core, the control rod drive mechanisms being provided above the control rod guide tubes and inside the reactor core shroud.
According to this invention, the core fuel is located at the RPV bottom portion and the control rod guide tubes are located above the core, and therefore the chimney effect provides strong natural circulation driving force to get maximum performance of the natural circulation reactor. Moreover this configuration is very compact and has economical advantage without re-circulation pumps.
In the boiling water reactor there may be provided a boiling water reactor characterized in that the pressure suppression pool is positioned higher than said reactor core, said pressure suppression pool being connected to said nuclear reactor pressure vessel by means of gravity-based piping through which the cooling water drops by gravity.
According to this invention, if gravity driven core cooling system as a passive safety system is adopted, then the water source of the gravity driven core cooling system is stored in the suppression pool, and hence pressure containment vessel volume can be reduced in compact.
In the boiling water reactor nuclear power plant there may be provided a boiling water reactor nuclear power plant characterized in that a piping and nozzles connected to the nuclear reactor pressure vessel are located above the reactor core.
According to this invention, since all obstacles such as nozzles, pipes or the like can be eliminated from the area below the nuclear reactor pressure vessel, the volume of the lower dry well can be minimized, so that it is possible to minimize the pool volume that is to be filled by the gravity driven core cooling system, and retention of the molten core material inside the nuclear reactor pressure vessel in order to prevent an event (severe accident) from progressing, can be performed readily as a severe accident situation countermeasure.
In the boiling water reactor nuclear power plant there may be provided a boiling water reactor nuclear power plant characterized in that a valve which can be optionally opened to an exterior of the core shroud is provided at a position above the fuel assembly.
According to this invention, a nuclear reactor pressure vessel internal valve which can be opened optionally to the outside the core shroud is provided in a location of the required height of the chimney portion, for example, in the shroud head above the fuel, and by opening this valve in the event of ATWS (Anticipated Transient Without Scram), a flow rate of the natural circulation can be reduced, so that the plant power can be suppressed until the boric acid solution injection system for shutting down the nuclear reactor is initiated.
In the boiling water reactor nuclear power plant there may be provided a boiling water reactor nuclear power plant characterized in that the walls of the pressure containment vessel are made from multiple steel plates having ribs, which are mutually opposing in a separated fashion, so that cooling means is formed by using the spaces between these multiple steel plates as water flowing passage or air flowing passage.
According to this invention, by constituting the walls of the nuclear reactor containment vessel by a ship hull type dual-steel-plate structure, and providing spaces in these dual-steel-plate walls which can be used for cooling the nuclear reactor containment vessel, then it becomes possible to cool the nuclear reactor containment vessel by only natural forces using the water or air passing through these spaces.
In the boiling water reactor nuclear power plant there may be provided a boiling water reactor nuclear power plant characterized in that the pressure suppression pool and the lower portion of the dry well are connected by means of a plurality of emergency piping, the piping being disposed at different positions in elevation level.
According to this invention, two coupling pipes (two communication pipes), for example, an upper and lower coupling pipe, are provided between the pressure suppression pool and the lower portion of the dry well, for causing natural circulation of pool water between the pool and the dry well, whereby, after flooding the lower portion of the dry well in the event of an accident, the heat released to the dry well is transferred to the pressure suppression pool by the natural circulating convection of the pool water, so that the heat can be effectively removed from inside the nuclear reactor containment vessel in an efficient manner.
In the boiling water reactor nuclear power plant there may be provided a boiling water reactor nuclear power plant characterized in that a normal use cooling system is connected to the space regions formed between the multiple steel plates.
According to this invention, by filling the spaces in the dual-steel-plate sections with water and connecting to the normal use cooling water system, it becomes possible to cool the inside the dry well during normal operation of the plant.
In the boiling water reactor nuclear power plant there may be provided a boiling water reactor nuclear power plant characterized in that a normally closed water drain pipe (discharge pipe) is led from the pressure suppression pool into the dry well at the base region of the nuclear reactor pressure vessel, the drain pipe is normally closed by a sealing device while the sealing device for this water drain pipe can be opened or released by heat sensing means in case of an emergency so as to open the water drain pipe.
According to this invention, even if a reactor core meltdown occurs, it is possible to cool and separate the molten material without needing to provide active components inside the containment vessel, and furthermore, any abnormal conditions can be detected reliably.
Moreover, in this invention, in addition to connecting the dry well space at the base of the nuclear reactor pressure vessel with the pressure suppression pool by means of piping, it is also preferable to seal the opening of the piping outlet of this pipe to the dry well space by means of a low-melting-point alloy such as Ag brazing material, solder or the like, and to provide a differential pressure meter in such a manner that the pressure differential in the piping can be measured.
In the boiling water reactor nuclear power plant there may be provided a boiling water reactor nuclear power plant characterized in that a heat pipe capable of exchanging heat is provided at a portion between the pressure suppression pool and the lower region of the dry well.
According to this invention, by connecting the dry well space at the base of the nuclear reactor pressure vessel to the pressure suppression pool by means of the heat pipe, heat released into the dry well in the event of an accident can be transferred to the pressure suppression pool by the natural circulation convection of the heat pipe, and hence the heat inside the nuclear reactor containment vessel can be effectively removed in an efficient manner.
In the boiling water reactor nuclear power plant there may be provided a boiling water reactor nuclear power plant characterized in that the nuclear reactor containment vessel is formed as a dual-cylinder structure wherein the dry well and the pressure suppression pool is positioned on the outer side of the dry well, in addition to which a guard pipe extending from the dry well section to the pressure suppression pool is provided, and piping and valves led from the nuclear reactor pressure vessel are accommodated inside this guard pipe.
According to this invention, since the nuclear reactor containment vessel has a dual-cylinder structure wherein the dry well is disposed on the inner side and the pressure suppression pool is disposed on the outer side thereof, and a nuclear reactor system is adopted wherein the active components positioned inside the dry well are kept to a minimum, the pipes such as the main steam pipe led form the nuclear reactor pressure vessel and the nuclear reactor containment vessel inside located valves or the like are accommodated inside the guard pipe extending from the dry well section of the dual-cylinder structure to the exterior thereof, thereby enabling required maintenance of valves or the like, to be performed inside the guard pipe.
In the boiling water type nuclear power plant a boiling there may be provided water type nuclear power plant characterized in that turbine system is located at an upper portion of the nuclear reactor building.
According to the present invention, the spent fuel pool and its associated peripheral apparatus are eliminated, and the turbine system or the like is located at the upper portion of the nuclear reactor building, thereby permitting the whole plant apparatus to be housed in a single module building.
In the boiling water reactor nuclear power plant there may be provided a boiling water reactor nuclear power plant characterized in that a removing space for accommodating the nuclear reactor pressure vessel is provided above the nuclear reactor pressure vessel in the nuclear reactor building.
According to this invention, the removing space is provided above the nuclear reactor pressure vessel, whereby the nuclear reactor pressure vessel including the dry well cylinder section can be exchanged in an integral way.
In the boiling water reactor nuclear power plant there may be provided a boiling water reactor nuclear power plant characterized in that the nuclear reactor building is located on a foundation having a seismic structure.
According to this invention, by locating the integrated nuclear reactor building module on a foundation having a seismic structure, it becomes possible to achieve standardized design of the building and the components and apparatus.
The invention provides a method for constructing a boiling water reactor nuclear power plant characterized in that the boiling water reactor nuclear power plant may be previously fabricated in a factory as building modules, then the modules are transported to a construction site, and only the required number of modules are installed so as to construct entire plant.
According to this invention, when the integrated building modules are fabricated in a factory, then transported to the construction site, and the plant output power can be selected as desired level by only installing the required number of modules at the construction site.